JP3374006B2 - Bearing steel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bearing steel suitable for use as an element member of rolling bearings such as roller bearings and ball bearings, and particularly to cold workability and low rolling fatigue life characteristics. Bearing steel.

[0002]

2. Description of the Related Art Bearing parts used in industrial machines and automobile parts are SU specified in JIS G4805.
J2 typified by C: 0.95 to 1.10 wt% and Cr: 1.30 to 1.60
High carbon chromium bearing steels containing wt% are most commonly used. This high carbon chrome bearing steel is melted and then rolled at a high temperature of about 1250 ° C. for about 30 hours at a high temperature for a long period of diffusion annealing to be rolled into a steel bar having a predetermined size. Further, in order to finish the bearing component, spheroidizing annealing is performed, followed by forming processing such as cutting, cold working or warm working, and then quenching and tempering. Here, the purpose of the diffusion annealing is to dissipate the giant carbide formed by combining carbon, chromium, etc., which occurs during melting and has a bad influence on the rolling fatigue life. The purpose of the spheroidizing annealing is to reduce the extremely high hardness in the as-rolled state due to the high carbon concentration, and to facilitate various subsequent processing. Further, quenching and tempering are performed to ensure the hardness and toughness required for the rolling bearing.

[0003]

By the way, in recent years, bearing parts which have been formed by cutting have been subjected to more severe working such as cold forging in order to improve the yield of steel products. In addition to the merit of improving the yield, these severe cold forming processes tend to increase particularly in terms of energy consumption reduction, working environment improvement, and dimensional accuracy improvement. When a conventional high carbon chrome bearing steel is used when performing severe working such as cold forging, cracking occurs during cold forging, which causes a problem in workability such as restriction in cold working. In addition, diffusion annealing of high temperature and long time is indispensable because of the dissipation of huge carbides caused by the components, which is a major factor of cost increase.

As a method for solving these problems, for example, the technique disclosed in Japanese Patent Laid-Open No. 2-54739 is disclosed. This method reduces cutting resistance or deformation resistance by reducing the carbon content of the material, so cold workability is generally improved to some extent compared to high carbon chrome steel, but depending on the composition range, Cracking still occurs even within the range of conventional cold forging of high carbon chromium steel. Further, no consideration is given to omitting diffusion annealing at high temperature for a long time. Further, in the method disclosed in Japanese Patent Application Laid-Open No. 1-127651, although the above-mentioned problem of diffusion annealing can be omitted, the problem of workability still remains. Therefore, an object of the present invention is to provide an inexpensive bearing steel that does not impair mechanical properties such as rolling fatigue life, has excellent cold workability, and does not require diffusion annealing.

[0005]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that spheroidizing annealing is required to improve workability in severe working such as cold forging. It was found that the structure has a great influence, and it was found that the workability such as cold forging can be improved by improving the spheroidized annealing structure. Further studies have shown that by setting the composition of the components within a specific range, the workability such as cold forging can be improved, and long-term diffusion annealing at high temperature can be omitted. We have developed an inexpensive steel for bearings, which has cold workability equivalent to or better than that of carbon chrome bearing steel. That is, the gist of the present invention is as follows.

(1) A bearing steel which is subjected to spheroidizing annealing of carbide and then subjected to cold working, wherein C: 0.71 wt.
% Or more , less than 0.80 wt%, Si: 0.50 wt% or more, 1.0 wt% or less, Mn: 0.10 wt% or more, 2.0 wt% or less, Cr: 0.43 wt% or less
Upper , 0.95wt% or less, Al: 0.050wt% or less, O: 0.0030wt
% Or less, with the balance being Fe and inevitable impurities, and having a characteristic that cracking does not occur at a cold forging compressibility of 60% or less, a steel for bearings having excellent cold workability.

(2) A bearing steel which is subjected to spheroidizing annealing of carbide and then subjected to cold working, wherein C: 0.71 wt.
% Or more , less than 0.80 wt%, Si: 0.50 wt% or more, 1.0 wt% or less, Mn: 0.10 wt% or more, 2.0 wt% or less, Cr: 0.43 wt% or less
Upper , 0.95wt% or less, Al: 0.050wt% or less, O: 0.0030wt
% Or less, and Ni: 0.10 wt% or more, 1.00 wt% or less,
Cu: 0.05 wt% or more, 0.50 wt% or less, Mo: 0.10 wt% or more,
It is characterized by containing one or more selected from 1.00 wt% or less, the balance being Fe and unavoidable impurities, and having a characteristic that cracking does not occur at a cold forging compressibility of 60% or less. Bearing steel with excellent cold workability.

(3) A bearing steel to be subjected to cold working after spheroidizing of carbide, C: 0.71 wt.
% Or more , less than 0.80 wt%, Si: 0.50 wt% or more, 1.0 wt% or less, Mn: 0.10 wt% or more, 2.0 wt% or less, Cr: 0.43 wt% or less
Upper , 0.95wt% or less, Al: 0.050wt% or less, O: 0.0030wt
% Or less, and Nb: 0.05 wt% or more, 0.50 wt% or less,
V: 0.05 wt% or more, 0.50 wt% or less, W: 0.05 wt% or more,
Characterized by containing one or more selected from 0.50 wt% or less, the balance being Fe and unavoidable impurities, and having a characteristic that cracking does not occur at a cold forging compressibility of 60% or less Bearing steel with excellent cold workability.

(4) A bearing steel which is subjected to spheroidizing annealing of carbide and then subjected to cold working, wherein C: 0.71 wt.
% Or more , less than 0.80 wt%, Si: 0.50 wt% or more, 1.0 wt% or less, Mn: 0.10 wt% or more, 2.0 wt% or less, Cr: 0.43 wt% or less
Upper , 0.95wt% or less, Al: 0.050wt% or less, O: 0.0030wt
% Or less, and Ni: 0.10 wt% or more, 1.00 wt% or less,
Cu: 0.05 wt% or more, 0.50 wt% or less, Mo: 0.10 wt% or more,
Contains one or more selected from 1.00 wt% or less, further Nb: 0.05 wt% or more, 0.50 wt% or less, V:
0.05wt% or more, 0.50wt% or less, W: 0.05wt% or more, 0.50
It is characterized by containing one or more selected from wt% or less, the balance consisting of Fe and unavoidable impurities, and having a characteristic that cracking does not occur at a cold forging compressibility of 60% or less. Bearing steel with excellent hot workability.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The bearing steel according to the present invention will be described in detail below. The inventors investigated the influence of the spheroidized annealing structure on the cold forgeability in order to improve the workability of the bearing steel such as cold forging. FIG. 1 shows the relationship between the critical compression rate during cold forging and the spheroidization rate of carbides from the above experiment. From FIG. 1, it was found that the cold forgeability was improved as the spheroidization rate was improved. Here, the spheroidization rate is defined as the ratio (%) of the total number of carbides having a vertical to horizontal ratio (aspect ratio) of 2 or less among the carbides present in a unit area. Therefore, it was found that a bearing steel having excellent cold workability can be obtained by improving the spheroidization rate.

As described above, in order to improve the spheroidizing rate, it is necessary to appropriately control the spheroidizing annealing conditions. Figure 2
Fig. 1 shows a schematic diagram of a general spheroidized annealing heat pattern. The inventor has conducted extensive research and studies on spheroidizing annealing conditions, and in order to improve the spheroidizing rate, the amount of chromium contained in the carbide before holding the maximum heating temperature of the spheroidizing annealing greatly affects the spheroidizing. I found that. That is, it has been clarified that the greater the amount of chromium contained in the carbide, the more the carbide nuclei that grow during the subsequent cooling remain, and the degree of spheroidization is improved. The reason for this is that as the amount of chromium contained in the carbide increases at the maximum heating temperature, the carbide is more difficult to dissolve, and particularly small nuclei of carbide tend to remain, and these nuclei grow and spheroidize during subsequent cooling. Seems to be. From the above examination results, it has been found that in order to improve the spheroidization rate of carbides, it is necessary to regulate the main constituent elements, carbon and chromium, in a specific range regarding the spheroidization annealing conditions. The reasons for limiting each component including these components will be described below.

C: 0.71 wt% or more and less than 0.80 wt% C is an important element in the present invention, and from the ratio with the amount of chromium, which is an element effective in improving cold workability and spheroidization rate, In addition, solid solution in the base to strengthen martensite,
It is necessary to add at least 0.71 wt% or more in order to improve strength, wear resistance and rolling contact fatigue life. On the other hand, if 0.80 wt% or more is added, cold workability, warm workability,
Machinability and toughness decrease, and diffusion annealing becomes impossible due to the relationship with other elements. Therefore, the C content is 0.71 ~
The range is less than 0.80 wt%.

Si: 0.50 wt% or more and 1.0 wt% or less Si is an element necessary as an element for improving the rolling fatigue life by forming a solid solution in the matrix in addition to deoxidizing. Content is 0.50wt%
If it is less than 1.0%, this effect is small. On the other hand, if it is added in an amount of more than 1.0% by weight, the hardness after spheroidizing is particularly increased, so that the machinability and the workability are significantly reduced. Therefore, the amount of Si is 0.50 to 1.
Limit to the range of 0 wt%.

Mn: 0.10 wt% or more and 2.0 wt% or less Mn improves the hardenability of steel, thereby increasing the toughness of the base martensite and effectively contributing to the improvement of rolling fatigue life. However, if it is less than 0.10 wt%, this addition effect is poor, while if it is added over 2.0 wt%, machinability,
Since the toughness and workability are significantly reduced, the Mn content is 0.10-
Limit to the range of 2.0 wt%. Incidentally, it is preferably 0.50 to 1.
It is preferable to set it in the range of 20 wt%.

Cr: 0.43 wt% or more and 0.95 wt% or less Cr is a particularly important element in the present invention. It is an element effective not only for improving the hardenability of steel and improving the strength and toughness of the matrix, but also for improving the spheroidization rate, which is closely related to the improvement of cold workability. If the content is less than 0.43 wt%, these effects are small. On the other hand, if the content exceeds 0.95 wt%, the diffusion annealing cannot be omitted due to the relationship with other elements. It should be noted that such an effect of Cr is almost saturated at 0.80 wt%, and at 0.80 wt% or more, due to the relationship with other elements, particularly the amount of C and the amount of Si, giant carbides are likely to be formed during melting. Were it to Tsu, Cr addition amount, 0.43 ~0.95wt%, preferably in the range of 0.43 ～0.80Wt
The range is%.

Al: 0.050 wt% or less Al is added as a deoxidizing agent, but since it combines with O to form a hard oxide inclusion, the rolling fatigue life is shortened. Therefore, the lowest possible value is desirable, 0.050
The upper limit is wt%.

O: 0.0030 wt% or less O combines with Al to form a hard oxide-based non-metallic inclusion, which reduces the rolling fatigue life. Therefore, it is desirable that the amount is as small as possible, and the upper limit is 0.0030 wt%.

The basic components have been described above, but in the present invention, one or two selected from Mo, Ni and Cu are further used.
1 or more selected from the above and / or Nb, V, W
One kind or two or more kinds can be added. The preferable addition amount range of each element and the reason for limitation are as follows. Ni:
0.10wt% or more, 1.00wt% or less, Cu: 0.05wt% or more, 0.50
wt% or less, Mo: 0.10 wt% or more, 1.00 wt% or less Mo, Ni, and Cu are all useful elements that enhance hardenability and improve rolling fatigue life of steel. However, Mo, Cu
If the amount of Ni is too large, the machinability of the steel is deteriorated, and if the amount of Ni is too large, not only the retained austenite is formed in a large amount to decrease the hardness of the steel material but also to reduce the rolling fatigue life. It also reduces the machinability of steel. Therefore, these elements are also added within the above-mentioned ranges in which there is no fear of this being involved.

Nb: 0.05 wt% or more, 0.50 wt% or less, V: 0.
05wt% or more, 0.50wt% or less, W: 0.05wt% or more, 0.50wt
% Or less Nb, V and W all combine with C in steel to improve wear resistance and effectively contribute to the improvement of rolling fatigue life and toughness due to the refinement of crystal grains. However, if any of the elements is too large, not only the carbide stabilizes at a high temperature, the hardness of the steel material decreases, the rolling fatigue life decreases, but also the machinability of the steel decreases. Therefore, these elements are also added within the above-mentioned ranges in which there is no fear of this being involved.

Next, a method for producing the steel of the present invention will be described. The steel of the present invention may be melted by any method such as a converter or an electric furnace, and the slab may be manufactured by either continuous casting or ingot manufacturing. Further, the hot rolling condition and the spheroidizing annealing condition are not particularly limited, and may be performed according to a conventional method.

[0021]

EXAMPLES The present invention will be described below based on examples.
The steel materials shown in Table 1 were melted in a converter, made into a steel piece by a continuous casting method, and then rolled into a steel bar having a diameter of 65 mm. Steel material No. 2
Regarding No. 1, steel pieces were similarly similarly made into steel pieces, further subjected to diffusion annealing at 1250 ° C. for 30 hours, and after disappearance of huge carbides, rolled into 65 mmφ steel bars. Next, 18 mmφ test pieces were cut out from the diameter / 4 portions of these steel bars, and the following spheroidizing annealing was performed in the air atmosphere to evaluate the cold forgeability. Further, after spheroidizing annealing, quenching and tempering were performed, and samples for evaluating rolling fatigue life were taken.

The heat treatment conditions for spheroidizing annealing, quenching, and tempering are as follows.・ Spheroidizing annealing: 750 ℃ × 2 hours → furnace cooling to 650 ℃,
After that, air cooling, quenching, tempering: 850 ℃ × 1 hour / water cooling, 180 ℃ ×
1 hour / air cooling Further, the test methods for cold forgeability and rolling fatigue life are as follows.・ Cold forging test: 15mm from spheroidized sample
A φ × 20 mm test piece is cut out, and the compression ratio is in the fully restrained state.
Crack occurrence rate at 50-70% is measured. ・ Rolling fatigue life: Hertz maximum contact stress: 600kgf / mm ² , Cyclic stress number: Approx.
The test was conducted under the condition of 500 cpm, and the test results were summarized on the probability paper as the ones according to the Weibull distribution, and the relative evaluation was made with the B ₁₀ life of steel material No. 21 (cumulative damage probability: total load count until peeling at 10%) as 1. Table 2 shows the measurement results of these cold forgeability and rolling fatigue life.

[0023]

[Table 1]

[0024]

[Table 2]

As is clear from Table 2, steel materials Nos. 22 and 23 having a C content lower than the range of the present invention are slightly improved in cold workability, but have a rolling fatigue life of No. 21 conventional. Material
It was only 0.7 and 0.8 times. It can be seen that the steel materials No. 24 and 25 having a Cr content lower than the range of the present invention have low spheroidization rate after spheroidizing annealing and cracking occurs at a relatively low compression rate, and cold workability is low. . Steel Nos. 26 and 27 having Si contents lower than the range of the present invention are improved in cold workability, but rolling fatigue life is 0.8 times and 0.9 times that of No. 21 conventional material.
It was only twice. Steel with C content higher than the range of the present invention
Steel No. 3 with No. 28, 29 and Cr content higher than the range of the present invention
0 means that the rolling fatigue life is 0.7 times that of the No. 21 conventional material,
Only 0.6 times and 0.5 times. From the microstructure observation result of the rolling fatigue life test piece, it was assumed that the life was short because of the huge carbide formed during melting.

In comparison with these comparative examples, steel materials No. 1 to No. 20
In each of the invention examples, no cracks were generated up to a compression rate of 60%,
Remarkably superior to conventional materials. In addition, the rolling fatigue life is 1.3 to 3.1 times that of the conventional material. Further, as shown in the above examples, addition of one or more of (Ni, Cu, Mo) and / or (Nb, V, W) does not impair cold workability and causes rolling fatigue. From the fact that the life is improved, it is understood that the combination can be freely added depending on the purpose of use.

[0027]

As described above, according to the present invention, the cold workability can be effectively improved without impairing the rolling fatigue life. It is possible to process up to the range that has occurred. Therefore, according to the present invention, the cutting process can be omitted or simplified, and the material yield and productivity can be improved. Further, according to the present invention, not only the alloy cost is reduced by reducing C and Cr, but also the diffusion annealing conventionally performed for dissipating the giant carbide generated during casting is omitted. Since it is possible, the cost of the bearing material can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a spheroidization rate of a carbide and a critical compression rate.

FIG. 2 is a schematic diagram showing a temperature pattern of spheroidizing annealing heat treatment.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiyuki Hoshino 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Prefecture (no address) Kawasaki Steel Works, Ltd. Mizushima Works (72) Inventor Shinichi Amano, Kawashima-dori, Kurashiki-shi, Okayama Prefecture 1 chome (no address) Kawasaki Steel Co., Ltd. Inside Mizushima Works (56) Reference JP-A-3-254341 (JP, A) JP-A-3-254340 (JP, A) JP-A-2-274837 (JP, A) JP-A-2-54739 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. A bearing steel to be subjected to cold working after spheroidizing of carbide, wherein C: 0.71 wt% or more and less than 0.80 wt%, Si: 0.50 wt% or more, 1.0 wt. % Or less, Mn: 0.10 wt% or more, 2.0 wt% or less, Cr: 0.43 wt% or more , 0.95 wt% or less, Al: 0.050 wt% or less, O: 0.0030 wt% or less, and the balance Fe and unavoidable Bearing steel with excellent cold workability, which is made of impurities and has the characteristic that cracking does not occur at cold forging compressibility of 60% or less. 2. A bearing steel to be subjected to cold working after spheroidizing of carbide, wherein C: 0.71 wt% or more and less than 0.80 wt%, Si: 0.50 wt% or more, 1.0 wt. % Or less, Mn: 0.10 wt% or more, 2.0 wt% or less, Cr: 0.43 wt% or more , 0.95 wt% or less, Al: 0.050 wt% or less, O: 0.0030 wt% or less, and Ni: 0.10 wt% or more , 1.00 wt% or less, Cu: 0.05 wt% or more, 0.50 wt% or less, Mo: 0.10 wt% or more, 1.00 wt% or less, and the rest contains Fe and unavoidable. Made of impurities, cold forging compression rate 60%
A bearing steel having excellent cold workability, which is characterized in that cracking does not occur in the following. 3. A bearing steel to be subjected to cold working after spheroidizing of a carbide, wherein C: 0.71 wt% or more and less than 0.80 wt%, Si: 0.50 wt% or more, 1.0 wt. % Or less, Mn: 0.10 wt% or more, 2.0 wt% or less, Cr: 0.43 wt% or more , 0.95 wt% or less, Al: 0.050 wt% or less, O: 0.0030 wt% or less, and Nb: 0.05 wt% or more , 0.50 wt% or less, V: 0.05 wt% or more, 0.50 wt% or less, W: 0.05 wt% or more, 0.50 wt% or less, and one or more selected from the rest, the balance Fe and unavoidable Made of impurities, cold forging compression rate 60%
A bearing steel having excellent cold workability, which is characterized in that cracking does not occur in the following. 4. A bearing steel to be subjected to cold working after spheroidizing of carbide, wherein C: 0.71 wt% or more and less than 0.80 wt%, Si: 0.50 wt% or more, 1.0 wt. % Or less, Mn: 0.10 wt% or more, 2.0 wt% or less, Cr: 0.43 wt% or more , 0.95 wt% or less, Al: 0.050 wt% or less, O: 0.0030 wt% or less, and Ni: 0.10 wt% or more , 1.00 wt% or less, Cu: 0.05 wt% or more, 0.50 wt% or less, Mo: 0.10 wt% or more, 1.00 wt% or less, and one or more selected, and Nb: 0.05 wt% Or more, 0.50 wt% or less, V: 0.05 wt% or more, 0.50 wt% or less, W: 0.05 wt% or more, 0.50 wt% or less, and the balance contains Fe and unavoidable Cold forging compression rate of 60%
A bearing steel having excellent cold workability, which is characterized in that cracking does not occur in the following.